Scavenge porting system

ABSTRACT

In a two cycle internal combustion engine of the type which utilizes the underneath side of the power piston as a scavenge pump piston, the improvement which consists of extra height, piston valved scavenge ports which are additionally valved by reed valves located in the transfer passageway close to the scavenge ports, in order to increase the scavenge gas mass flow through capability of the cylinder, improve the scavenge gas flow pattern, or both.

United States Patent m1 Tenney h SCAVENGE PORTING SYSTEM [76] Inventor:William L. Tenney, Crystal Bay,

[58] Field of Search 123/73 R, 73 PP, 73 AA,

123/73 AV, 73 AC, 73 AD, 73 AB, 73 EF, 73 B, 73 BA, 73 C, 73 CA, 73 CB,73 CC, 73 D, 73 DA, 73 E, 73 A; 137/512.15

, [56] References Cited UNITED STATES PATENTS 1,979,770 1 1/1934 VPollister -.l23/65 V 1,986,630 H1935 Fowler 123/73 AA 3,168,890 2/1905Eilert 123/73 AB 3,612,014 10/1971 Tenney 123/73 R FOREIGN PATENTS ORAPPLICATIONS 95,602

OPTIONAL RESONANT OR PRESSURE WAVE TUNED EXHAUST SYSTEM 5/1939 Swedenl23 /73A [451 June 11, 1974 254,704 5/1927 Great Britain 123/65V 181,04511/1935 Switzerland. 123/65 V 16,482 9/1912 Denmark [23/73 BA 4/1936Germany 123/65 V Primary Examiner-Wendell Burns Attorney, Agent, orFirm-Bugger, Johnson &

Westman ABSTRACT In a two cycle internal combustion engine of the typewhich'utilizes the underneath side of the-power piston as a scavengepump piston, the improvement which consists of extra height, pistonvalved scavenge ports which are additionally valved by reed valveslocated in the transfer passageway close to the scavenge ports, in orderto increase the scavenge gas mass flow through capability of-thecylinder, improve the scavenge gas flow pattern, or both.

10 Claims, 6 Drawing Figures PATENTEDJM 1 1 1914 31815; 558

SHEET 1 BF 3 FIG. .2

OPTIONAL RESONANT OR PRESSURE WAVE TUNED EXHAUST SYSTEM SCAVENGE PORTINGSYSTEM BACKGROUND OF THE INVENTION TERMINOLOGY In describing two cycleengines, terminology can be confusing as regards intake port, inletport, scavenge port, transfer port, etc. In particular, the terms inletport and intake port are used to describe both the power cylinderscavenge ports and the scavenge pump inlet ports. Also, the termtransfer port is used to describe both the power cylinder scavenge portand the port at the opposite end of the transfer passageway opening intothe scavenge pump system. Confusion is the result. To reduce suchconfusion, for the purposes of this patent the terminology will beemployed as follows:

The last port through which the scavenge and charging medium passes onits way into the power cylinder, will be referred to as a scavenge port.

the passageway which connects each scavenge port with the scavenge pumpwill be referred to as a transfer passageway.

a port located at the opposite end of each transfer passageway from thescavenge port, which port connects the transfer passageway with thescav-' enge pump system at the opposite end from the scavenge port,willbe referred to as a crankcase outlet port.

Any port which opens from atmosphere into the scavenge pumpsystem tofeed or charge the scavenge pump will be referred to as an intake portor inlet port.

Also, the terminology upper dead center, lower dead center, top deadcenter, bottom dead center, upp er.or top cylinder or crankcase portionor area, bottom or lower cylinder or crankcase portion, etc., will beused in this patent to refer to an engine so oriented as to have thecylinderlongitudinal axis located in a generally vertical plane with thecylinder head uppermost or on top, and the crankshaft and crank chamberlocated in an area generally directly underneath or at the lower orbottom end of the cylinder.

FIELD OF THE INVENTION The field of the invention is that of the twocycle, reciprocating piston type internal combustion engine having thescavenge ports located in that section of the cylinder wall situatednearest to the bottom dead center position of the power piston head, andutilizing the underside of thepower piston as a pumping piston for thescavenge pump system.

DESCRIPTION OF THE PRIOR ART enge gas mass flow through capability,improving the scavenge gas flow pattern within the power cylinder, orboth.

' US. Pat. No. 3,046,958 issued July 31', 1962 to F. N. Bard et al.discloses an internal combustion percussive device based on a' two cycleengine in which the energy imparted to'the piston by combustion of thefuel-air charge is transmitted to the work load by percussion of thepiston directly against a tool or anvil. In the Bard et al patent thescavenge port opening and closing in the power cylinder wall iscontrolled by the reciprocating movement of the piston head timing edgein cooperation with the adjacent piston side wall area, and additionalcontrol of the transfer passageway opening to the scavenge port and tothe scavenge pump syst'emis exercised by means of areed type valvelocated in the transfer passageway. The system as illustrated anddescribed, however, neither servesto increase the scavenge gas massflow'through capability of the cylinder, not to provide an efficientscavenge gas flow pattern.

As illustrated, in Bard et al the height of the scavenge port opening,as measured from the upper edge to the lower edge of the port, amountsto only about 14 percent of the piston stroke as scaled from thedrawing. However, the upper edge of the scavenge port is located abovethe timing edge of the piston head (with the piston in bottom deadcenter position) by a distance equal to approximately 28 percent of thepiston stroke. Thus half of the potential gas flow area availablebetween the timing edge of the piston head with the piston at the bottomdead center position, and the upper edge of the scavenge port, iswasted. The scavenge gas flow through capability is thereby decreasedinstead of increased. Further, the longitudinal sectional view in Bardet a]. does not illustrate either the scavenge or exhaust port extendingany distance radially around the cylinder wall. 'Thus the usualperipheral area around the cylinder wall. available for-scavenge andexhaust ports is not illustratedas being used. Again the gas flowthrough capability of the ports and cylinder appears the opposite ofbeing increased or maximized. Finally,'there is no teaching in thedescription that even hints at increasing or maximizing the gas flowthrough capability.

Also, the scavenge gas flow pattern through the power cylinder bore ofBard et al is not improved in any way by the illustrated arrangement,but is in fact very inefficient. The scavenge gas flow is aimed directlyacross the cylinder toward the exhaust port, which will result in verypoor scavenging and maximum short cir-' cuiting of the fresh charge outof the cylinder via the exhaust port Even the domed portion of thepiston head. does not project into or tend to interrupt the straightline scavenge gas flow path leading directly from the scavenge portacross to the exhaust port.

The Bard et al patent thus offers'no teaching as to increasing ormaximizing the scavenge gas mass flow through capability of the portsystem or of providing somewhat unusually high scavenge port whichscales on the drawing at a height of some 29 percent of the stroke.However, the corresponding depth of transfer passageway illustratedscales at only some 17 percent of the stroke, which would tend to negateany flow rate advantage of the high port. Finally, FIG. 9 of Gommelquite clearly illustrates that when the reed valves are in the fullyopened position the flow through the open ends of the opposing valves isnot only obstructed by head on bucking of the gas streams one against;the other, but also by blocking of the transfer passageway cross sectionby the open reed valves, with the result that the gases can flow onlyvia a tortuous and ob.- structed path that serves toreduce the mass rateof scavenge gas flow instead of increasing it. Also, there is no hintgiven in thedescription of any aim'or attempt toward increasing ormaximizing the rate of gas flow.

As to improving or making more efficient the scavenge gas flowpatternwithin the cylinder, nothing in either FIGS. '7, 8 and 9 or inthe accompanying description of Gommel suggests such an outcome.

' SUMMARY or THE INVENTION The invention consists of the use of extraheight scavenge ports, as described and illustrated in my said copendingapplication Ser. No. 224,756, filed Feb. 9, 1972, but with the necessaryadditional valving control supplied by reed valves located in thetransfer passageway(s)' close to the extra height ports, instead of bypiston side wall ports. The reed valves are located gento the closedposition on their seats without undue shock or stress, resulting inmaximum reed life. The extra height scavenge ports, additionallycontrolled by reed valves, are provided in order to increase the massrate of gas flow capability out of the piston underside scavenge pumpsystem and thus increase the mass air flow through capability of thecylinder with resulting increased power output'capability. The extraheight scavenge ports-may also be utilized to provide a more efficientscavenge gas flow pattern in the work cylinder spent gas content, as byreleasing the scavenge gases into the cylinder at a point nearer to the.cylinder head area or by providing a scavenge air stream of desirablylarger cross-sectional area in relation to the cylinder borecross-sectional area. 1

in particular, the extra height scavenge ports of the invention areuseful in short stroke engines having cylinder bore to stroke ratiosgreater than in the 1:1 to 1.25:1 ratio range which has usually producedthe highest power output per cubic inch of power piston displacement inprior art engines which have been limited to use of normal heightscavenge ports only. in such enin even less favorable poweroutputtogether with poorer thermal efficiency particularly in the case of fuelmixture scavenged engines. The additional scavenge port flow areaafforded by the use of extra height scavenge ports overcomes thesedifficulties inherent in the usual short stroke engines. I

It is of course understood that cylinder exhaust flow capability andalso scavenge pump system intake flow capability must be designed tocomplement the increased scavenge port system flow capability madepossible by use of extra height scavenge ports, if maximum engine powerincrease due to increased air handling capability is to be realized.However, suitable complimentary increases in exhaust flow capability andscavenge pump system inlet flow capability can usually be made withoutdeparting from the general teachings of the present state of the art. Aspecial form of the exhaust port used for increasing exhaust flowcapability in combination with aparticula'rly'popular and usefulscavenge port arrangement which may include extra height scavenge portsis described and illustrated in my copending application Ser. No.233,588, filed Mar. 10, 1972, entitled Two Cycle Engine with AuxiliaryExhaust Ports.

The additional scavenge port timing control offered by the reed valvesof the subject invention as opposed to the additional piston side wallcontrol port valving of my copending application Ser. No. 224,756justifies the slight mechanical complexity added by thereed valvesystem, for several reasons'First, the reed valves provide a variablevalve timing automatically responsive to chanically late closure whichwould otherwise results in undue loss of fresh charge gases out of thecylinder during the port closing phase of piston motion. Sucharrangements work well as long as the engine rpm does not fall muchbelow the rpm-tuned range of the exhaust system. However when the enginerpm falls too low,the exhaust pressure wave enters the cylinder when thescavenge ports are still open to the scavenge pump system, resulting atthe least in interference with transfer of fresh charges betweenscavenge pump and power cylinder. Contaminated hot gases can also beblown back into the scavenge pump system with known results as todeterioration of engine operation. Finally,

particularly when piston valved scavenge pump intake systems areutilized in combination with fuel-air mixture intake devices, extremelyover-rich fuel-air mixtures result and cause objectionable irregularengine running, misfiring, etc. This result occurs because when tremeover-rich mixture condition with attendant misfiring, 'sparkplugfouling, and other objectionable operating conditions. The variabletiming provided by the reed valves tends to act as a check valve toprevent reverse flow through the transfer passageways under suchconditions, resulting in greatly improved engine operation.

Another advantage of the reed valve additional control is that it tendsto reduce the gas volume trapped in the transfer passagewaybetween theclosed additional valving device and the scavenge port. Particularlywhen rpm-tuned exhaust pressure waves are utilized to bring aboutearlier effective exhaust port closure, an additional portion of thecylinder fresh charge gases may be forced back into the partof thetransferpassageway located between the additional control valve cutoffpoint and the scavenge port outlet, and thus result in some reduction ofthe fresh gas charge mass trapped within the power' cylinder bore.Withthe reed valve(s) placed close to the extra height scavenge ports,thepassageway valve available for such blow back of fresh charge volumecan be reduced in comparison to the transfer passageway volume extendingall the way down to a piston side wall control port.

Another advantage of the reed valve type additional extra heightscavenge port control system is that it checks reverse flow of heatedcombustion gases before it can reach the piston skirt which is alreadysubjected to high thermal loading and consequent marginal lubricationconditions. With the piston skirt side wall control port, of coursethere is no mechanical barrier which intervenes between hot combustiongases flowing back down into the transfer passageway and the pisto'nskirt or side wall. f

It is an object of the present invention to provide increase over thenormal flow capability available through prior art scavenge port systemshaving only normal height scavenge ports. It is also an object of theinvention to make possible a scavengegas flow pattern of improvedefficiency.

These objects are achieved in the. disclosed embodiments of theinvention through the use of extra height scavenge ports with reedvalves located close to the associated ports and which are designed andoriented so as to minimize flow eddies, sudden'changes in flowdirection, and flow resistance through the transfer passageways.

The illustrated reed valve and extra height scavenge ports areespecially desirable when used in combination with short stroke engines.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical sectional viewtaken as on line 1-1 of FIG. 3 illustrating a two cycle engine havingextra height scavenge ports and utilizing reed valves in the transferpassageways associated with these extra height scavenge ports inaccordance with the present invention;

FIG. 2 is a fragmentary vertical sectional view taken as on line 2-2 inFIG. 3; 7

FIG. 3 is a transverse sectional view taken-as on line 3-3 in FIG. I;

FIG. 4 is a fragmentary vertical sectional view of a modified form ofthe present invention in an engine having a l.5:l bore to stroke ratio;

FIG. 5 is a fragmentary sectional view taken as on line 5-5 in FIG. 4';and 1 FIG. 6 is a fragmentary vertical sectional view of a furthermodified scavenge port and transfer passageway configuration.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 11, 2 and 3,a two cycle internal combustion engine illustrated generally at 10includes a cylinder block ll having a cylinder bore 12 defined therein.A piston 13 is mounted for reciprocating movement i'n-said cylinderbore. The piston is connected' with a piston pin to a connecting rod 14,which in turn is connected at its'lower end to a'crankshaft 15 mountedin a crankcase 16. The crankcase defines an interior chamber l7 which inthe form of the invention shown is utilized as a scavenge pump chamber.The underside of the piston 13 is open to the chamber 17 and acts as ascavenge pump piston. i

The fresh charge gas inlet to the crank chamber 17 can be through anydesired .valving arrangement, for example reed valves or. rotary valves,or as shown, an inlet port 20 which is valved bythe lower portions ofthe side wall of the piston 13 may be used as the inlet valvearrangement. A suitable connecting tube 21 leads to a carburetor orother suitable charge inlet device.

The cylinder has an exhaust port 22 leading to an exhaust pipe 23 thatin turn can be connected to a suitable exhaust system 24 such as aresonant or pressure wave tuned'exhau'st. system. i

A plurality of scavenge ports are defined in the cylinder wall and thesescavenge ports are connected by transfer passageways to the crankchamber 17. The scavenge ports as shown include an extra height oppositescavenge port 25 located intha t portion of the cyl- I position'of thepiston .timing edge 13A. The bottom inder wall opposite from the exhaustport 22. The extra height scavenge port 25 is connected to a transferpassageway 26. The transfer passageway 26, as shown, has

through the transfer. passageway 26 into the cylinder bore in anupwardly directed flow path toward the distal end of the cylinder, or inother words, away from the end of the cylinder adjacent the .bottom deadcenter dead center position of the piston timing edge is on a level inthe cylinder with the bottom line 22A of the exhaust port 22 in-the formshown. The extra height ports have an upper edge at least substantially30 percent of the piston stroke as measured from a reference leveldefined by the piston timing edge in bottom dead center position of thepiston to the distal edge of the port. As shown herein, the extra heightports havedistal edges spaced from the reference level a distance equalto substantially 45 percent of the piston stroke. I

The transfer passageway 26 which connects to the extra height oppositescavenge port'25 is opento the crank chamber I7 through a crankcaseoutlet port 28. The extra height scavenge port 25 is covered and uningportions of. the cage as shown in solid lines in FIG.

1 and serve to close the internal passageways through the case. Thereeds 33 are made of springy or resilient material, forexample springsteel, and whenever the gas pressure at .the scavenge port and in theupper portion of transfer passageway 26 is sufficiently less than thepressure at the crankcase outlet port 28, the reeds 33 will move awayfrom their solid line position and open the passageways through cage 32.When the reeds 33 move to their dotted line positions shown in FIG. 1the reed valve assembly reaches its maximum opening. When closed, thereed .valve assembly 31 prevents reverse flow of gases from the scavengeport 25 through the transfer passageway 26 to the crankcase outlet port28. The reed valve assembly 31 is, as shown in'FlG. 3, of sufficientlateral width to have two reeds 33 on each side of the valve cage.

When port 25 is uncovered during a power stroke of the piston andpressure in-cylinder bore 12' has blown down through the exhaust port 22to a level sufficiently different from the pressure-in the scavengepumpsystern the reeds 33 will move away from their seats on cage 32.Whenthe valve ass'emblyis at its maximum opening, the reeds are in theirdotted linepositionssupported against the'curved back stop surfacesindicated at 34 and 35. Due to pressure differential between the chamber17 and bore 12 fresh charge gases will flow from thecrankcase outletport 28 through the openings in the cage 32 and then out through theupper portions of the transfer passageway 26 and scavenge port 25 intothe cylinder bore 12.

As shown perhaps best in FIG. also has a pair of side extra heightscavenge ports 40 defined therein. The side extra height scavengeports40 are positioned, as shown, alongside the opposite scavenge port 25 andare located above,'i.e. spaced toward the distal end of the cylinderfrom, a pair of side normal height scavenge ports 41 provided in thecylinder wall. The extra height side'scavenge ports 40 open to transferpassageways 42 defined in the cylinder block. Thetransfer passageways 42at their lower ends terminatein crankcase outlet ports 43 that are opento the chamber 17 of the scavenge pump system. As shown, the distal orupper edges of the extra height side scavenge ports 40 are substantiallyon the same level in the cylinder bore 12asthe upper edge of the'extraheight opposite scavenge port 25. The lower edgesof ports40 are definedby the wallsseparating the ports 40 from normal height ports 41.

The reed valveassembly 31 comprises an additional valving means for theextra height scavenge port 25 apart from the, piston 13. The piston 13will uncover the upper portions of the extra height scavenge port 25during the power stroke before the pressure in cylinder bore 12 hasblown down to, a, suitable level for scavenging, and the reed valveassembly 31 will remain closed to prevent blowback into the scavengepump system- The reeds automatically open when the pressure in thecylinder bore drops a sufficient amount in relation'to the pressure inchamber .17. Downward piston movement increases the'pressure of thefresh charge gases which weredrawn into chamber 17 through port 20as thepiston previously moved toward top dead center position.

The reed valve assembly 31 is held in place by means of suitable capscrews. A cover plate 36 is alsofastened in place by cap screws to coverthe opening the cylinder block used for installing or removing the reedvalve assembly 31. Thecover plate may also be used to exert clampingpressure on the fixed ends of the reeds. The reed valve assembly 31 isselected in size so as to provide a maximum flow area at least equal toand preferably greater than theminimum area of the transfer pas- A reedvalve assembly 44 is mounted in transfer passageway 42. The assembly 44includes [first and second reed valve assembly portions 45 and 46 andassociated reeds 47 and 48. The reed valve assembly portions 45 and 46define interior flow passageways as shown, that are closed by thevalve'reeds or leaves 47 and 48 with the reeds in their normal solidline position shown in FIG. 2. The reeds, when closed, seat againstsurfaces is sufficiently different from the pressure in theupperportions of the transfer passageway 42 (above the valve assembly44) the valve reeds 47 and 48 will be moved toavalve open position andfresh charge gases will flow through the passageways 42 and out throughports 40. The. reeds automatically open in response-to pressuredifferentials, and likewise also automatically close to prevent reverseflow in direction from the extra height scavenge ports 40 toward thecrankcase outlet ports 43 through the passageway 42.

When the reeds 47-and 48 are fully opened as shown in dotted lines theyare supported on curved surfaces that correspond generally'to the normalcurvature of the reeds in their open positions. The support surface forreed 48 is an outer surface of valve-assembly portion 45, and thesupport surface for reed 47 is one of the surfaces defining passageway42. The support surfaces for the reeds are oriented so as to provide asmooth gas flow path toward the scavenge port 40. The internal surfaces45A and 46A of the passageways through the valve assembly portionsgenerally conform to the curvature of the supporting surface for thereeds in the reed open positions and contribute further toward a smoothgas flow path toward scavenge port 40. 1

The normal height scavenge ports 41, which are below the extra heightscavenge ports 40, open to I 2,-the cylinder wall Scavenge ports 54'opento transfer passageways 55 which open directly into the chamber 17through crankcase outlet ports 56. The normal height scavenge ports 54are also valved in the usual manner by the piston; The portion ofv thecylinder wall below ports 41 which separate the piston from transferpassageway 52 can be removed if desired without affecting operation ofthe engine.

As shown, the valve assembly portions 45 and 46 are held in place in thecylinder block with suitable cap screws, and are inserted or removedthrough a provided opening leading to their respective transferpassageway 42. The cover for this provided opening is formed as part ofvalve assembly portion 46.

The reed valve assembly 44 provides a maximum flow area through thevalve at least equal to and preferably greater than the area of scavengeport 40. Also, the reed valve is designed and oriented so as to providea smooth, even and direct gas flow path leading to scavenge port 40.

While two different forms of reed'valves have been shown, of courseother types of reed valves can be utilized. It should be noted that thereed valves in the present devices are located in the transferpassageways close to the extra height ports in order'to minimize thevolume of the transfer passageway between the reed valve and theassociated scavenge port.

The engine shown in FIGS. 1, 2 and 3 has a 1:1 bore to stroke ratio, andin FIGS. 4, 5 and 6, an engine having a 1.521 bore to'stroke ratio isillustrated. As has already been explained, the use of extra heightscavenge ports is especially advantageous in engines utilizing high boreto stroke ratios, such as 1.521.

Referring to FIG. 4, the engine shown fragmentarily has a cylinder block65, with an internal cylinder bore 66 in which a piston 67 is slidablymounted for reciprocating movement. The piston is connected to aconnecting rod (not shown) inthe normal manner, and many of theconventional details are omitted from FIG. 4. Bottom deadcenter positionof the piston is reached when the piston head timing edge 67A is at thesame level as the lower edge 68A of exhaust port 68. The upper deadcenter position of timing edge 67A coincides with the edge A of themating surface between the cylinder block 65 and the cylinder head. I

In this form of the invention, the cylinder has'an exhaust port 68leading therefrom, and as shown in FIG. 4 a first pair of extra heightside scavenge ports 69 are provided in the cylinder. The extra heightside scavenge ports 69 extend from a level along the level of the pistontiming edge 67A inbottom dead center position of the piston continuouslyup to their upper or distal.

edge, as shown substantially 45 percent of the piston stroke. The 'ports69 open to transfer passageways 70 that in turn open through crankcaseoutlet ports 71 to a chamber 72 forminga part of the scavenge pumpsystem. The underside-of the piston 67' again is used as a of the pistontiming edge in bottom dead center position a distance equal toapproximately percent of the piston stroke.

The extra height side scavenge ports 69 and 73 and their associatedtransfer passageways as shown are positioned so that the walls of thetransfer passageways adjacentthe respective ports are aimed indirections such' that the flow from the respective scavenge ports isintended to provide for efficient scavenging in the known manner wherebythe gas flow streams from the opposed pairs of side scavenge ports meetand combine toform a single gas stream which proceeds upwardly along thesection of cylinder wall located opposite from exhaust port 68.

As illustrated in FIG. 4, a reed valve assembly is mounted in thetransfer passageway '70. In this particular instance, the reed valveassembly 80 includes three valve assembly portions 81, 82 and 83defining flow passageways, and having valve reeds or leaves 84, and 86,respectively, associated therewith so as to close the valve passagewayswhen the reeds are in their solid line positions. The valve assemblyportions are held in place with suitable cap screws.

The sum of the cross-sectional areas of the individual flow channelsthrough the valve assembly portions is greater than the cross-sectionalarea of the transfer passageway at the scavenge port 69. Thus the reedvalve assemblyfitl constitutes a minimal obstruction to flow offreshchargegases through the transfer passageway 70 during engineoperation.

'The transfer passageway wall 89 has a curved surface that forms abackstop for the valve reed 84 in its'full open position, and theadjacent valve assembly portions 81 and 82 form backstop surfaces forthevalve reeds 85 and 86 so that the reeds will be supported when in theirrespective fullopen positions. Likewise, the surfaces defining the flowpassageways through the respective valve assembly portions aresodesigned and oriented as to direct the fiow through the valve assemblyin a desired smooth flow path through the transfer passageways with aminimum of sudden changes indirection, turbulenceor disrupting eddies.Thereed valve assembly 80 is' located in the transfer passageway closeto the scavenge port in order that the volume in the passageway, betweenthe reed valve assembly and its associated scavenge port is kept to adesirable minimum. I

, After the piston 67 uncovers the extra height scavengeport 69 duringthe powerstroke, the reed valve assembly 80 will remain closeduntil thepressure in the.

cylinder bore 66 has blow down via exhaust port 68and the pressure atthe crankcase outlet port 711 is sufficiently different fromthe pressureat the extra height scavenge port 69 to cause the reeds to open. Whenthereeds open fresh charge gases will flow from the scavenge pump systeminto the cylinder because of the pressure differential between thescavenge pump and I the cylinder bore.

In FIG. '5, it can be seen that the transfer passageway 74 has a reedvalve assembly 90 therein which is constructed in a substantiallyidentical manner to the .reed

1 l enge port arrangement of FIG. 6 includes an extra height scavengeport 91, and an underlying normal height scavenge port 92. These ports91 and 92 are separated by wall 93. The extra height scavenge port 91opens to a transfer passageway 94' that communicates with crankcaseoutlet port 95. A reed valve assembly 96 is locatedin transferpassageway 94. Reed valve assembly 96 is a single reed valve assemblyhaving a reed 97, which, when fully open, is supported back against thesurface of the wall 93 as shown in dotted lines. The reed valve assemblyis fastened into the cylinder block with suitable cap screws. Normallythe reed is in its solid line closed position. It is of course to beunderstood that this modified form is not limited to the illustratedsingle reed valve assembly; mulitple reed valves may also be used.

The normal height scavenge port 92 opens to a transfer. passageway 98that communicates with a crankcase outlet port 99 leading to thescavenge pump chamber 72 (when used with the engine of FIG. 4). Thevalve of normal height scavenge port 92 is controlled in the normalmanner by the Fresh charge gases will not flow from the scavenge pumpsystem chamber 72 to the cylinder 66 through the port-91 until thepressure in the cylinder-has blown down through the exhaust port to alevel which permits the reed 97'to move to openposition. The valve reed97 automatically closes to prevent flow from the cylinder back into thescavenge pump system. The reed valves are automatic, pressure responsivevalves, so that as soon as the pressure in the cylinder bore has blowndown to a suitable level the valves will automatically open and permitfresh charge gases to flow into the cylinder bore.

The use of reed valves has been illustrated in combination with engineshaving two different particular piston 67jand'the piston timing edgebore to stroke ratios, but of course the reed valves can be used withthe extra'height scavenge ports regardless of the bore to stroke ratios.

Also, it should be noted that the type of engine ignition does notaffect the operation of the invention, in that the engine can have sparkignition or can be a compression ignition or a glow plug ignitionengine; Fuel injection through the cylinder head (or elsewhere also canbe utilized if desired; The scavenge pump intake valve system also canbe of any desired type.

Two types of extra height scavenge ports have been shown. in one type,the extra height scavenge port is superposed over a normal heightscavenge port. The extra height port has an additional valving meanscomprising the reed valve, while the normal height port is valved onlyby the piston. The transfer passageway for theunderlying normal heightport is therefore unobstructed. All reed valves provide some flowobstructiornso fora minimum of obstruction the use of conventionalpiston valved, normal height scavenge ports underlying the extraheightports is preferred. This configuration also reduces the size andcomplexity of the reed valves necessary because the reed valves onlycon- -trol the flow through the extra height ports.

On the other'hand, the type of extra height port that is continuous fromthe bottom dead center level of the piston timing edge up to the distaledge of the port and which usesone large reed valve eliminates the needfor height ports. lt should also be noted that with the reed valveassembly used for the continuous port, (such as reed valve assembly 80)the valve at all times protects against reverse flow from the cylinderthrough the transfer passageway and into the scavenge pump system. v

The extra height scavenge ports tend to improve the scavenge gas flow.pattern within the cylinder because the scavenge gases emanate fromtheports closer to the distal end of the cylinder, thereby aiding inclearing the spent combustion gases from this end of the cylinder. Aspreviously mentioned, the extra height scavenge ports also tend toprovide for a greater cross-sectional area of the scavenge gas flowstream, which is particularly important in short stroke engines.

in all forms of the invention (utilizing the extra height scavengeports) the scavenge ports are substantially continuous or open from theupper edges of the extra height ports down to the level-of thepistonhead timing edge with the piston in bottom dead center position,in

order to provide maximum utilization of the cylinder wall area availablefor scavenge ports and thus aid in maximizing the scavenge gas flowthrough capability. The divider walls used in some of the illustratedforms of the invention for dividing the scavenge ports-into extra heightports with underlying normal height ports do not occupy any substantialamount of cylinder wall area that could otherwise be used for scavengeport area.

a divider wall to separate the transfer passageways, as

is needed with separated extra height and normal It should also be notedthat in all forms of the inventionthe reed valve assemblies are locatedclose to the extra height scavenge ports and are so oriented anddesigned as to provide a smooth gas flow path directed toward theassociated scavenge ports with a minimum of changes in flow-directionand hence a minimum of turbulence and flow disrupting eddies.

Finally, the embodiments shown have the upper edges of the extra heightscavenge ports terminating on a level in the cylinder no higher than thelevel of the upper or distal edge of the exhaust port. This insures thatthe exhaust gas pressure and temperature towhich the reeds are subjectedwill not be high enough to cause damage to the reeds.

What is claimed is:

' l. A two cycle engine having a cylinder and a power piston witha'piston head timing edge and an underside,

" cated in a first portion of the cylinder adjacent a reference linealong the piston timing edge with the piston in bottom deadcenterposition, said cylinder having a distal end towards which thepiston moves during the compression stroke, transfer passageway meansleading from said scavenge pump system to the scavenge ports, saidpassageway means being elongated and extending close to the cylinder andgenerally in the same direction as the axis of said cylinder from theunderside of 'said piston to the scavenge ports, said scavenge ports insaid cylinder extending from substantially adjacent said reference linetoward the distal end of the cylinder and being covered and uncovered bysaid piston timing edge as the piston reciprocates in the cylinder,significant portions of at least some of said scavenge ports beinguncovered by said piston when the distance between the piston timingedge and the reference line is not less than substantially 25 percent ofthe piston stroke, and reed valve means in said transfer passage 13 waymeans for said somescavenge ports to substantially prevent reverse flowfrom said cylinder to said scavenge pump system, and to permit flow fromsaid scavenge pump system into'the associated transfer passageway meansand then to said some scavenge ports in response to pressuredifferentials, said reed valve means extending generally longitudinallyof said passageway means and including at least one reed member having asupported lower end and a pressure responsive upper end that is movablegenerally transverse of said passageway means to an open position topermit flow from the scavenge pump system, and means positioning saidupper movable end substantially above the bottom of the piston when thelatter is in bottom dead center position so that said upper movable endis adjacent said scavenge port means, said reed member in an openposition guiding flow toward the associated scavenge ports.

2.The combination as specified in claim l wherein said reed valve meanscomprises surface means forming a smoothly curved surface to guide flowfrom said scavenge pump system to the associated scavenge port means,said reed member being positioned to move in direction to guide flowtoward said scavenge port means as the reed valve member opens.

3. The combination as specified in claim 1 wherein said reed valvemember comprises a blade like valve member movable from a closed to anopen position, and said transfer passageway means is defined by wallsurfaces in said engine,'a portion of said wall surfaces beingpositioned adjacent said reed valve means and shaped to support and stopat least one of said blade like valve members in its open position in ashape conforming to the general curve of the blade like member inanunsupported open position.

4. The combination as specified in claim 1 wherein said reed valve meanscomprises a body member mountedin said passageway means, and having aplurality of fiow channels through said body member, separate blade likeresilient members positioned adjacent each of said channels and movableto a closed position to prevent reverse flow through the associated flowchannels and yieldingly movable under pressure differentials to an openposition to permit flow from the scavenge pump system to said scavengeport means.

5. The combination as specified in claim 1 wherein the reed valve meansincludeflow passageways having a total effective flow path crosssectional area with the reed valve means fully open that is at leastequal to the effective minimum flow path cross sectional area throughthe associated transfer passageway means without the reed valve means inthe associated transfer passageway means.

6. The combination as specified in claim 1 wherein at least one of saidsome scavenge ports extends from a distal edge thereof continuouslytoward a level corresponding to said reference line.

7. The combination as specified in claim ll wherein said cylinder hasport means including first scavenge port means of no greater than normalheight and second extra height scavenge port means, said first scavengeport means opening to separate transfer passageway means from thetransfer passageway means for the second scavenge port means, said firstscavenge port means being positioned between said second scavenge portmeans and said reference line, communication between the scav'enge' pumpand the cylinder through said first scavenge port means being controlledby the piston and the piston head timing edge, and wherein said reedvalve means is associated with the second scavenge port means and notwith said first scavenge port means.

8. The combination as specified in claim 7 wherein said first and secondscavenge port means at least partially overlap in circumferentialrelationship.

9. The combination as specified in claim 1 and in which there areseparate passageway means open to separate portions of said somescavenge port means to divide at least some of said scavenge port meansinto separate portions which at least partially overlapcircumferentially in the cylinder, said reed valve means beingpositioned in at least some of said separate transfer passageways.

10. The combination as specified in claim 1 wherein said cylinder isdefined by a wall having a circumference, an exhaust port leading fromsaid cylinder and defined through said wall, and said scavenge portsbeing defined in said wall to occupy a major portion of thecircumference. of said wall adjacent said reference line other than theportion of the circumference adjacent the reference line occupied bysaid exhaust port.

1. A two cycle engine having a cylinder and a power piston with a pistonhead timing edge and an underside, and which utilizes the underside as apiston of a scavenge pump system and which has scavenge ports located ina first portion of the cylinder adjacent a reference line along thepiston timing edge with the piston in bottom dead center position, saidcylinder having a distal end towards which the piston moves during thecompression stroke, transfer passageway means leading from said scavengepump system to the scavenge ports, said passageway means being elongatedand extending close to the cylinder and generally in the same directionas the axis of said cylinder from the underside of said piston to thescavenge ports, said scavenge ports in said cylinder extending fromsubstantially adjacent said reference line toward the distal end of thecylinder and being covered and uncovered by said piston timing edge asthe piston reciprocates in the cylinder, significant portions of atleast some of said scavenge ports being uncovered by said piston whenthe distance between the piston timing edge and the reference line isnot less than substantially 25 percent of the piston stroke, and reedvalve means in said transfer passageway means for said some scavengeports to substantially prevent reverse flow from said cylinder to saidscavenge pump system, and to permit flow from said scavenge pump systeminto the associated transfer passageway means and then to said somescavenge ports in response to pressure differentials, said reed valvemeans extending generally longitudinally of said passageway means andincluding at least one reed member having a supported lower end and apressure responsive upper end that is movable generally transverse ofsaid passageway means to an open position to permit flow from thescavenge pump system, and means positioning said upper movable endsubstantially above the bottom of the piston when the latter is inbottom dead center position so that said upper movable end is adjacentsaid scavenge port means, said reed member in an open position guidingflow toward the associated scavenge ports.
 3. The combination asspecified in claim 1 wherein said reed valve member comprises a bladelike valve member movable from a closed to an open position, and saidtransfer passageway means is defined by wall surfaces in said engine, aportion of said wall surfaces being positioned adjacent said reed valvemeans and shaped to support and stop at least one of said blade likevalve members in its open position in a shape conforming to the generalcurve of the blade like member in an unsupported open position.
 4. ThecombinaTion as specified in claim 1 wherein said reed valve meanscomprises a body member mounted in said passageway means, and having aplurality of flow channels through said body member, separate blade likeresilient members positioned adjacent each of said channels and movableto a closed position to prevent reverse flow through the associated flowchannels and yieldingly movable under pressure differentials to an openposition to permit flow from the scavenge pump system to said scavengeport means.
 5. The combination as specified in claim 1 wherein the reedvalve means include flow passageways having a total effective flow pathcross sectional area with the reed valve means fully open that is atleast equal to the effective minimum flow path cross sectional areathrough the associated transfer passageway means without the reed valvemeans in the associated transfer passageway means.
 6. The combination asspecified in claim 1 wherein at least one of said some scavenge portsextends from a distal edge thereof continuously toward a levelcorresponding to said reference line.
 7. The combination as specified inclaim 1 wherein said cylinder has port means including first scavengeport means of no greater than normal height and second extra heightscavenge port means, said first scavenge port means opening to separatetransfer passageway means from the transfer passageway means for thesecond scavenge port means, said first scavenge port means beingpositioned between said second scavenge port means and said referenceline, communication between the scavenge pump and the cylinder throughsaid first scavenge port means being controlled by the piston and thepiston head timing edge, and wherein said reed valve means is associatedwith the second scavenge port means and not with said first scavengeport means.
 8. The combination as specified in claim 7 wherein saidfirst and second scavenge port means at least partially overlap incircumferential relationship.
 9. The combination as specified in claim 1and in which there are separate passageway means open to separateportions of said some scavenge port means to divide at least some ofsaid scavenge port means into separate portions which at least partiallyoverlap circumferentially in the cylinder, said reed valve means beingpositioned in at least some of said separate transfer passageways. 10.The combination as specified in claim 1 wherein said cylinder is definedby a wall having a circumference, an exhaust port leading from saidcylinder and defined through said wall, and said scavenge ports beingdefined in said wall to occupy a major portion of the circumference ofsaid wall adjacent said reference line other than the portion of thecircumference adjacent the reference line occupied by said exhaust port.